The key unit is a kind of element constituting a mobile device and has a multiplicity of keys (push buttons) for operating switches, respectively, assembled and arranged on the surface of a sheet. Each key is composed of a key pad which is made of a soft rubber or a thermoplastic elastomer and a key top which is made of a rigid material and is attached onto the surface of the key pad. The key pad has a projection for pressing the switch (so-called “pusher”) formed on the rear face. The keys in each horizontal row are connected to each other through the key pad. Since each key is illuminated by a light source provided under the key pad (referred to as “illuminated key”), light-transmitting materials are used for the key top and the key pad. An illuminated key switch is fabricated if a circuit board containing light sources and having switch elements arranged to oppose the respective switch-pressing projections of the associated keys is brought into intimate contact with the lower face of the thus constituted key unit.
When an illuminated key unit is to be produced, key tops and a key pad are formed in separate steps respectively, and then they are assembled by bonding them together with an adhesive. Alternatively, key tops are formed first and are inserted into an injection mold to form a key pad by means of insert molding. It is necessary to plot on the key tops characters, symbols, etc. representing the functions of the respective keys. This step is referred to as “marking.” Generally, marking is applied, for example, to the upper face (referred also to as “top face”) or lower face (bottom face) of each key top, or inside the key top. When marking is to be applied to the top face, measures should be taken to secure abrasion resistance of the characters, symbols, etc. marked on the key top. On the other hand, the key tops may be opaque, and marking can be carried out after bonding of the key tops to the key pad. When marking is to be applied to the bottom face, it is essential that the key top is transparent and that marking should be completed before the key tops are bonded to the key pad. To that end, the content of the characters, symbols, etc. to be marked must be defined before bonding of the key tops to the key pad.
While mobile devices including portable telephones are shipped to different language regions of all over the world, the days counting from determination of the destination to shipping is extremely limited. Of a number of elements constituting a mobile device, those elements excluding the key unit which are not related to the language used in the destination can be assembled before the destination is determined. However, in the case of key unit, although some markings including those of numerical characters which are not related to the relevant language can be applied beforehand, it is impossible to apply final marking and complete production of the key unit in the state where the language is not determined and there is still any unconfirmed element in the content of the characters, symbols, etc. to be marked. Nevertheless, it is too late to start production of key units after determination of the destination, whereas stock production of key units in many different languages beforehand leads to glut, disadvantageously.
The problems described above are only some of those generally observable in the system of build-to-order manufacturing, and there are some conventional technologies directed to solving similar problems. For example, in the patent literatures listed below, Patent Literature 1 relates to curtailment of lead time; Patent Literature 2 relates to information service of delivery schedule according to few-day base; and Patent Literature 3 relates to utilization of parts lists per model:    Patent Literature 1:    Unexamined Japanese Patent Publication (Kokai) No. 2002-202805    Patent Literature 2:    Unexamined Japanese Patent Publication (Kokai) No. 2002-244711    Patent Literature 3:    Unexamined Japanese Patent Publication (Kokai) No. 2003-015722
A first object to be attained by the present invention is to provide a key top, which permits setting up of the content of characters, symbols, etc. to be plotted on the key top at any time without being subject to limitations of other steps of key unit assembly and which is marked with characters, symbols, etc. such that they have sufficient abrasion resistance and also to provide a marking method for such purposes.
A second object to be attained by the present invention is to find out a material for forming key tops of illuminated key units, which can impart, when marking is applied to the top face of each key top, sufficient abrasion resistance to characters, symbols, etc. marked on the top face of the key top, as well as, to provide a marking method for such purposes and also to find out a method of marking the key top.
A third object to be attained by the present invention is to find out a process for producing an illuminated key unit, which enables completion of the key unit as soon as possible once the destination is determined.